Patient Dose Distributions Research Articles

Comparison of 3D and 2D gamma passing rate criteria for detection sensitivity to IMRT delivery errors

This study compared three‐dimensional (3D) and two‐dimensional (2D) percentage gamma passing rates (%GPs) for detection sensitivity to IMRT delivery errors and investigated the correlation between two kinds of %GP. Eleven prostate IMRT cases were selected, and errors in multileaf collimator (MLC) bank sag, MLC leaf traveling, and machine output were simulated by recalculating the dose distributions in patients. 2D doses were extracted from the 3D doses at the isocenter position. The 3D and 2D %GPs with different gamma criteria were then obtained by comparing the recalculated and original doses in specific regions of interest (ROI), such as the whole body, the planning target volume (PTV), the bladder, and the rectum. The sensitivities to simulated errors of the two types of %GP were compared, and the correlation between the 2D and 3D %GPs for different ROIs were analyzed. For the whole‐body evaluation, both the 2D and 3D %GPs with the 3%/3 mm criterion were above 90% for all tested MLC errors and for MU deviations up to 4%, and the 3D %GP was higher than the 2D %GP. In organ‐specific evaluations, the PTV‐specific 2D and 3D %GP gradients were −4.70% and −5.14% per millimeter of the MLC traveling error, and −17.79% and −20.50% per percentage of MU error, respectively. However, a stricter criterion (2%/1 mm) was needed to detect the tested MLC sag error. The Pearson correlation analysis showed a significant strong correlation (r > 0.8 and P < 0.001) between the 2D and 3D %GPs in the whole body and PTV‐specific gamma evaluations. The whole‐body %GP with the 3%/3 mm criterion was inadequate to detect the tested MLC and MU errors, and a stricter criterion may be needed. The PTV‐specific gamma evaluation helped to improve the sensitivity of the error detection, especially using the 3D GP%.

Image-based Data Mining to Probe Dosimetric Correlates of Radiation-induced Trismus

To identify imaged regions in which dose is associated with radiation-induced trismus after head and neck cancer radiation therapy (HNRT) using a novel image-based data mining (IBDM) framework. A cohort of 86 HNRT patients were analyzed for region identification. Trismus was characterized as a continuous variable by the maximum incisor-to-incisor opening distance (MID) at 6 months after radiation therapy. Patient anatomies and dose distributions were spatially normalized to a common frame of reference using deformable image registration. IBDM was used to identify clusters of voxels associated with MID (P ≤ .05 based on permutation testing). The result was externally tested on a cohort of 35 patients with head and neck cancer. Internally, we also performed a dose-volume histogram-based analysis by comparing the magnitude of the correlation between MID and the mean dose for the IBDM-identified cluster in comparison with 5 delineated masticatory structures. A single cluster was identified with the IBDM approach (P < .01), partially overlapping with the ipsilateral masseter. The dose-volume histogram-based analysis confirmed that the IBDM cluster had the strongest association with MID, followed by the ipsilateral masseter and the ipsilateral medial pterygoid (Spearman's rank correlation coefficients: Rs = -0.36, -0.35, -0.32; P = .001, .001, .002, respectively). External validation confirmed an association between mean dose to the IBDM cluster and MID (Rs = -0.45; P = .007). IBDM bypasses the common assumption that dose patterns within structures are unimportant. Our novel IBDM approach for continuous outcome variables successfully identified a cluster of voxels that are highly associated with trismus, overlapping partially with the ipsilateral masseter. Tests on an external validation cohort showed an even stronger correlation with trismus. These results support use of the region in HNRT treatment planning to potentially reduce trismus.

Inter-patient image registration algorithms to disentangle regional dose bioeffects

Radiation therapy (RT) technological advances call for a comprehensive reconsideration of the definition of dose features leading to radiation induced morbidity (RIM). In this context, the voxel-based approach (VBA) to dose distribution analysis in RT offers a radically new philosophy to evaluate local dose response patterns, as an alternative to dose-volume-histograms for identifying dose sensitive regions of normal tissue. The VBA relies on mapping patient dose distributions into a single reference case anatomy which serves as anchor for local dosimetric evaluations. The inter-patient elastic image registrations (EIRs) of the planning CTs provide the deformation fields necessary for the actual warp of dose distributions. In this study we assessed the impact of EIR on the VBA results in thoracic patients by identifying two state-of-the-art EIR algorithms (Demons and B-Spline). Our analysis demonstrated that both the EIR algorithms may be successfully used to highlight subregions with dose differences associated with RIM that substantially overlap. Furthermore, the inclusion for the first time of covariates within a dosimetric statistical model that faces the multiple comparison problem expands the potential of VBA, thus paving the way to a reliable voxel-based analysis of RIM in datasets with strong correlation of the outcome with non-dosimetric variables.

Comparison of penumbra regions produced by ancient Gamma knife model C and Gamma ART 6000 using Monte Carlo MCNP6 simulation

The accuracy of penumbral measurements in radiotherapy is pivotal because dose planning computers require accurate data to adequately modeling the beams, which in turn are used to calculate patient dose distributions. Gamma knife is a non-invasive intracranial technique based on principles of the Leksell stereotactic system for open deep brain surgeries, invented and developed by Professor Lars Leksell. The aim of this study is to compare the penumbra widths of Leksell Gamma Knife model C and Gamma ART 6000. Initially, the structure of both systems were simulated by using Monte Carlo MCNP6 code and after validating the accuracy of simulation, beam profiles of different collimators were plotted. MCNP6 beam profile calculations showed that the penumbra values of Leksell Gamma knife model C and Gamma ART 6000 for 18, 14, 8 and 4 mm collimators are 9.7, 7.9, 4.3, 2.6 and 8.2, 6.9, 3.6, 2.4, respectively. The results of this study showed that since Gamma ART 6000 has larger solid angle in comparison with Gamma Knife model C, it produces better beam profile penumbras than Gamma Knife model C in the direct plane.

The comparison between 6 MV Primus LINAC simulation output using EGSnrc and commissioning data

AbstractIntroductionMonte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The purpose of this research is comparison between 6 MV Primus LINAC simulation output with commissioning data using EGSnrc and build a Monte Carlo geometry of 6 MV Primus LINAC as realistically as possible. The BEAMnrc and DOSXYZnrc (EGSnrc package) Monte Carlo model of the LINAC head was used as a benchmark.MethodsIn the first part, the BEAMnrc was used for the designing of the LINAC treatment head. In the second part, dose calculation and for the design of 3D dose file were produced by DOSXYZnrc. The simulated PDD and beam profile obtained were compared with that calculated using commissioning data. Good agreement was found between calculated PDD (1·1%) and beam profile using Monte Carlo simulation and commissioning data. After validation, TPR20,10, TMR and Spvalues were calculated in five different field.ResultsGood agreement was found between calculated values by using Monte Carlo simulation and commissioning data. Average differences for five field sizes in this approach is about 0·83% for Sp. for TPR20,10differences for field sizes 10×10 cm2is 0·29% and for TMR in five field sizes, the average value is ~1·6%.ConclusionIn conclusion, the BEAMnrc and DOSXYZnrc codes package have very good accuracy in calculating dose distribution for 6 MV photon beam and it can be considered as a promising method for patient dose calculations and also the Monte Carlo model of primus linear accelerator built in this study can be used as method to calculate the dose distribution for cancer patients.

Comparison of calculated dose distributions reported as dose-to-water and dose-to-medium for intensity-modulated radiotherapy of nasopharyngeal cancer patients

Advanced dose calculation algorithms for radiation therapy treatment planning can report external beam photon dose 2-sided, in terms of dose-to-medium (Dm) and dose-to-water (Dw). The purpose of our study was to determinate the effect of Dw and Dm reporting modes built in Elekta Monaco treatment planning system on intensity-modulated radiotherapy dose distributions for patients with nasopharyngeal cancer. For 13 patients involved in this retrospective study, 2 plans were created: 1 using Dw and another according to Dm reporting mode. Treatment plans were normalized such that 100% planning target volume should be covered by 95% of prescribed dose. Dose-volume constraints were assigned according to international standards. The comparison between dose distributions was performed evaluating quantities important for respective volumes of interest. For target volumes, heterogeneity index and conformity index methodology were used along with the maximum dose concept. Also, for the comparisons over particular organ at risk, maximum dose or mean dose as well as dose-volume concepts were used. For all target volumes and majority of organs at risk, the differences between 2 reporting modes are statistically insignificant, but this is not the case for bony structured organs at risks: mandible and cochlea. It was observed that Dw is higher than Dm with mean difference of 9.91% (p = 0.000009) of the mandible volume covered with 70 Gy. The same trend was observed for left and right cochlea with difference in mean dose of 8.74% (p = 0.037) and 6.87% (p = 0.029), respectively. The comparative analysis of dosimetric parameters in this study shows that the selection of reporting modes in Monaco treatment planning system can produce dose differences up to 15% in high-density volumes such as mandible and cochlea, which might have clinical consequences.

Evaluation and comparison of dose distributions for nasopharyngeal carcinoma patients treated by Jaws-Only IMRT technique and by 3D-CRT technique at Dong Nai General Hospital

The goal of radiation therapy is twofold: maximize the possibility of destroy malignant cells while minimizing the damage to healthy tissue. The introduction of intensity modulated radiation therapy (IMRT) technique has brought improvements in this goal. Multi-leaf collimator (MLC) is a useful tool for IMRT. However, the use of MLC is not necessarily mandatory. The Panther Treatment Planning System version 4.6, Prowess Inc., enables the implementation of this technique for accelerator without MLC (the socalled Jaws-Only IMRT technique). This study aims to evaluate the results of application of Jaws-only IMRT technique for nasopharyngeal carcinoma patients at Dong Nai general hospital. Twenty five patients were randomly selected for this study. For each patient, two plans were generated: 3D-CRT (Three-Dimensional Radiation Treatment) and JO-IMRT. The dose distributions, dose-volume histograms (DVH), conformity indexes (COIN), homogeneity indexes (HI) were used to compare between these two plans and find out the best plan. Pretreatment verifications were performed for all patients' plans using ion chamber (Farmer Type Chamber FC65-P, IBA), detector array (MapCHECK2, Sun Nuclear Corporation and Octavius 4D 1500, PTW). The average deviation between measurement and calculation for point dose was 2.3±1.1 %, within limit dose constraint. For detector array measurements, the gamma index with 3 % dose difference and 3 mm was higher than 95 %. The results showed that the JO-IMRT technique had generated better dose distribution in the target volume and reduced dose to healthy tissues compared to 3D-CRT.

Patient-specific dosimetry of 99mTc-HYNIC-Tyr3-Octreotide in children

BackgroundTechnetium-99m-hydrazinonicotinamide-Tyr3-octreotide (99mTc-HYNIC-TOC) is recognized as a promising radiopharmaceutical for diagnosing neuroendocrine tumors (NETs). However, 99mTc-HYNIC-TOC dosimetry has been investigated only for adults. As pediatric radionuclide therapies become increasingly common, similar dosimetric studies for children are urgently needed. The aim of this study is to report personalized image-based biodistributions and dosimetry evaluations for children studies performed using 99mTc-HYNIC-TOC and to compare them with those from adult subjects.Eleven children/teenage patients with suspected or diagnosed NETs were enrolled. Patient imaging included a series of 2–3 whole-body planar scans and SPECT/CT performed over 2–24 h after the 99mTc-HYNIC-TOC injections. The time-integrated activity coefficients (TIACs) were obtained from the hybrid planar/SPECT technique. Patient-specific doses were calculated using both the voxel-level and the organ-level approaches. Estimated children doses were compared with adults’ dosimetry.ResultsPathologic uptake was observed in five patients. TIACs for normal organs with significant uptakes, i.e., kidneys, spleen, and liver, were similar to adults’ TIACs. Using the voxel-level approach, the average organ doses for children were 0.024 ± 0.009, 0.032 ± 0.017, and 0.017 ± 0.007 mGy/MBq for the kidneys, spleen, and liver, respectively, which were 30% larger than adults’ doses. Similar values were obtained from the organ-level dosimetry when using OLINDA with adapted organ masses. Tumor doses were 0.010–0.024 mGy/MBq. However, cross-organ contributions were much larger in children than in adults, comprising about 15–40% of the total organ/tumor doses. No statistical differences were found between mean doses and dose distributions in patients with and without pathologic uptakes.ConclusionAlthough the children TIACs were similar to those in adults, their doses were about 30% higher. No significant correlation was found between the children’s doses and their ages. However, substantial inter-patient variability in radiotracer uptake, indicating disparity in expression of somatostatin receptor between different patients, emphasizes the importance and necessity of patient-specific dosimetry for clinical studies.

Abstract ID: 101 Comparison of dose calculation between AAA algorithm and Monte Carlo calculation for prostate cancer

In radiotherapy, the accuracy of dose calculation is very important for a treatment success. Analytical algorithms implemented in commercial Treatment Planning Systems (TPS) achieve this calculation. In order to reduce time calculation; these algorithms do some assumptions, which cause important errors, especially in the cases with high heterogeneities. The aim of our study is to evaluate the AAA analytical algorithm of dose calculation using a Monte Carlo method, which is the most accurate, and considered as a reference in dose calculation. To accomplish this purpose, we started by the modeling of the treatment machine (Varian 2100C LINear ACcelerator) for the photons mode and the energy of 18MV, using EGSnrc Monte Carlo code. After that, we used this model to calculate dose distribution in patients treated for prostate cancer. Finally, the results are compared to those of Eclipse AAA TPS.

Factors Associated With In-field Recurrence in Patients Treated With Liver SBRT

Stereotactic body radiation therapy (SBRT) is an emerging technique for treatment of primary and secondary liver malignancies. Reports in the literature of the impact of set up uncertainties and other factors influencing in-field recurrences are lacking. The aim of this study was to examine the magnitude and consistency of interfractional shifts, their association, and association of other factors with in-field recurrences in patients treated with liver SBRT. All patients treated with liver SBRT at a single institution between 2014 and 2016 with doses 40 – 50 Gy in 5 fractions were included. Tumor histologies included hepatocellular carcinoma (50%), cholangiocarcinoma (4%), liver metastases from colorectal (39%) and other cancers (7%). Median follow-up was 15 months. A bony match was performed using orthogonal kV images. A soft-tissue match was then performed using a cone beam CT (CBCT). The shift data included in this study are the shifts made after the CBCT. Respiratory motion management techniques included abdominal compression, respiratory gating or free breathing, with the target respiratory motion below 1 cm. No dedicated internal fiducial markers were used. Retrospective analysis of prospectively collected data was performed as a part of an institutional Quality Assurance process. A total of 46 patients met the eligibility criteria. Data on interfractional shifts were available for 22 patients. One- and two-year in-field control rates were 64% and 30%, respectively. Mean vector shift length was 0.37 cm (systematic error 0.14 cm, random error 0.29 cm), 86% of patients had no shifts greater than 1 cm, 96% of all the shifts performed were less than 1 cm. Inferior tumor location had lower interfractional shift values compared with other tumor locations (one sided t-test=1.81, p-value=0.047). Despite that, inferior tumor location was weakly associated with in-field recurrence on univariate analysis (HR=2.63, p-value=0.093; 95% CI 0.86, 8.06), with median time to in-field recurrence 11 months as opposed to 21 months in patients with other tumor locations. Liver metastasis histology was also weakly associated with in-field recurrence, compared with primary liver histologies (HR=1.63, p-value=0.087; 95% CI 0.93, 2.87). Radiation dose, motion management technique, planning target volume and interfractional shift magnitude were not associated with in-field recurrence. Set up uncertainty and reproducibility was acceptable and did not impact in-field recurrences. Cone beam CT with soft tissue matching appears to be an appropriate set-up verification method. Inferior tumor location was associated with higher in-field recurrences in this patient cohort, likely due to proximity of critical structures, such as small bowel, resulting in target volume coverage compromises. Detailed evaluation of dose distribution in patients with in-field recurrences is underway.

Establishment of a calibration curve for an isocentric cobalt unit using Monte Carlo simulation

Measurement of dose distribution in patients during radiotherapy is impossible. The Monte Carlo simulation is an alternative method for dose calculations. In routine radiotherapy, the source-to-surface distance (SSD) method is not practical for an isocentric unit because it requires numerous values of tissue–air ratios and inverse square law. Therefore, this method is time consuming. In this paper, the curves of relative depth doses were obtained for three different SSDs using the MCNP4C Monte Carlo simulation and approximated with a single curve called calibration curve. This curve was compared to the curve obtained by published data, differing in approximately 5% in the worst case. It was also observed that the obtained results were more accurate for distances between −5 and 10 cm from source-to-axis distance.

Impact of spot charge inaccuracies in IMPT treatments.

Spot charge is one parameter of pencil-beam scanning dose delivery system whose accuracy is typically high but whose required value has not been investigated. In this work we quantify the dose impact of spot charge inaccuracies on the dose distribution in patients. Knowing the effect of charge errors is relevant for conventional proton machines, as well as for new generation proton machines, where ensuring accurate charge may be challenging. Through perturbation of spot charge in treatment plans for seven patients and a phantom, we evaluated the dose impact of absolute (up to 5× 106 protons) and relative (up to 30%) charge errors. We investigated the dependence on beam width by studying scenarios with small, medium and large beam sizes. Treatment plan statistics included the Γ passing rate, dose-volume-histograms and dose differences. The allowable absolute charge error for small spot plans was about 2× 106 protons. Larger limits would be allowed if larger spots were used. For relative errors, the maximum allowable error size for small, medium and large spots was about 13%, 8% and 6% for small, medium and large spots, respectively. Dose distributions turned out to be surprisingly robust against random spot charge perturbation. Our study suggests that ensuring spot charge errors as small as 1-2% as is commonly aimed at in conventional proton therapy machines, is clinically not strictly needed.

Increasing the power of tumour control and normal tissue complication probability modelling in radiotherapy: recent trends and current issues

The release of a homogeneous high dose to the tumour region has been one of the cornerstones of radiotherapy (RT) treatment since its early days. According to the organ type and to the cancer histology, different doses are required in order to inactivate malignant cells, thus stopping proliferation. However, radiation-induced cell killing is a stochastic process. Tumour control probability (TCP) models have been developed in order to assign a success rate to a given RT treatment. At the same time, there is the need to keep the risks of normal tissue toxicity at an acceptable level. Normal tissue complication probability (NTCP) models provide a means of doing this. Traditionally, TCP and NTCP models combine clinical outcomes with dosimetric information in terms of dose-volume histograms (DVH). Model parameters are derived by mathematical fits to clinical observations and are subsequently used to estimate the risk of tumour relapse or toxicity. In both types of models, all of the patient dosimetric information is condensed into the DVHs, which represents a potential limitation on their descriptive and predictive power. This choice, related to historical and practical reasons, does not allow the full complexity of the 3D dose distribution in the patient to be taken into account. Neglecting these aspects might be relevant in a modern RT setting, which often includes the presence of high dose gradient regions. This has motivated research on ‘advanced’ TCP and NTCP models, able to tackle the problem by looking at a different scale, e.g. in tumour sub-regions or at the single voxel level. This is relevant not only from the purely dosimetric point of view. Increasing evidence is reported on the heterogeneity of cancer tissues, suggesting that non-uniform dose distributions could result in improved survival, for instance if targeted to take into account sub volumes with high clonogen density or hypoxic radioresistant regions. Similarly, radiation-induced side effects are part of a complex biological response, which depends not only on cell killing, but also on the inflammatory response and in some cases on the interplay among different organs. Obviously, conventional NTCP models cannot describe this scenario, and the development of more advanced mathematical tools is needed. This review will be focused on the discussion of recent studies showing possible directions for moving the field of TCP and NTCP modelling forward. Without diminishing the role and usefulness of available models, the aim is to shed light on the benefits that might be achieved by ‘enhanced’ modelling. This could represent an important step in the gradual transition of radiation therapy towards a form of precision medicine.

Determining the mechanical properties of a radiochromic silicone-based 3D dosimeter

New treatment modalities in radiotherapy (RT) enable delivery of highly conformal dose distributions in patients. This creates a need for precise dose verification in three dimensions (3D). A radiochromic silicone-based 3D dosimetry system has recently been developed. Such a dosimeter can be used for dose verification in deformed geometries, which requires knowledge of the dosimeter’s mechanical properties. In this study we have characterized the dosimeter’s elastic behaviour under tensile and compressive stress. In addition, the dose response under strain was determined. It was found that the dosimeter behaved as an incompressible hyperelastic material with a non-linear stress/strain curve and with no observable hysteresis or plastic deformation even at high strains. The volume was found to be constant within a 2% margin at deformations up to 60%. Furthermore, it was observed that the dosimeter returned to its original geometry within a 2% margin when irradiated under stress, and that the change in optical density per centimeter was constant regardless of the strain during irradiation. In conclusion, we have shown that this radiochromic silicone-based dosimeter’s mechanical properties make it a viable candidate for dose verification in deformable 3D geometries.

Methodological accuracy of image-based electron density assessment using dual-energy computed tomography.

Electron density is the most important tissue property influencing photon and ion dose distributions in radiotherapy patients. Dual-energy computed tomography (DECT) enables the determination of electron density by combining the information on photon attenuation obtained at two different effective x-ray energy spectra. Most algorithms suggested so far use the CT numbers provided after image reconstruction as input parameters, i.e., are imaged-based. To explore the accuracy that can be achieved with these approaches, we quantify the intrinsic methodological and calibration uncertainty of the seemingly simplest approach. In the studied approach, electron density is calculated with a one-parametric linear superposition ('alpha blending') of the two DECT images, which is shown to be equivalent to an affine relation between the photon attenuation cross sections of the two x-ray energy spectra. We propose to use the latter relation for empirical calibration of the spectrum-dependent blending parameter. For a conclusive assessment of the electron density uncertainty, we chose to isolate the purely methodological uncertainty component from CT-related effects such as noise and beam hardening. Analyzing calculated spectrally weighted attenuation coefficients, we find universal applicability of the investigated approach to arbitrary mixtures of human tissue with an upper limit of the methodological uncertainty component of 0.2%, excluding high-Z elements such as iodine. The proposed calibration procedure is bias-free and straightforward to perform using standard equipment. Testing the calibration on five published data sets, we obtain very small differences in the calibration result in spite of different experimental setups and CT protocols used. Employing a general calibration per scanner type and voltage combination is thus conceivable. Given the high suitability for clinical application of the alpha-blending approach in combination with a very small methodological uncertainty, we conclude that further refinement of image-based DECT-algorithms for electron density assessment is not advisable.

Risk of secondary cancers: Bridging epidemiology and modeling

Epidemiological studies of long term radiotherapy survivors provide useful insights into dose-response relationships for secondary cancer induction risk at high doses. There are uncertainties involved in estimating the dose to the location of the second malignancy, because the dose distributions in radiotherapy patients can be spatially highly heterogeneous and the size of the diagnosed tumor is on the order of a few cm. Therefor it is nearly impossible to obtain the exact dose corresponding to the exact tumor induction location and so organ specific dose-response relationships have large errors not only in the reported risk, but also in the estimated dose.In this work two alternative methods are proposed for future applications involving investigations into dose response relationships for second cancer induction risk, the method of organ sub-division and the method of risk equivalent dose. The method of organ sub-division takes the inevitable inhomogeneous dose distribution into account by applying epidemiological methods to organ sub-divisions which have a nearly homogenous dose. The method of risk equivalent dose combines risk modeling and epidemiological data analysis. Risk models can be optimized by using an iterative procedure assuming a variation of organ specific dose-responses.The advantage of the alternative methods is that the inhomogeneity of the dose in the organs at risk is taken into account. The second method has the additional advantage that the dose to the location of the tumor site must not be known and that epidemiologically obtained risks that were not stratified by organ specific risk can be used.

Diagnostic Reference Levels and Monitoring Practice Can Help Reduce Patient Dose From CT Examinations.

The purpose of this study is to establish provincial diagnostic reference levels (DRLs) and to determine whether this process may help reduce the patient radiation dose from the most frequently performed CT examinations. We investigated the following CT examinations: head, chest, low-dose chest, abdomen and pelvis, and chest, abdomen, and pelvis examinations. The sample for each protocol included 15 patients of average body weight (mean [± SD], 70 ± 20 kg). The differences in dose between scanners were evaluated using one-way ANOVA. Correlations between dose, scanner age, and the number of detector rows were assessed using the Pearson correlation coefficient. A sample of abdominal and chest examinations were randomized and blinded for review by experienced radiologists who graded diagnostic image quality. Provincial DRLs were calculated as the 75th percentile of patient dose distributions. For hospitals with doses exceeding the DRLs, dose reduction was recommended, followed by another survey. The initial survey included data of 1185 patients, and an additional 180 patients were surveyed after protocol optimization. The differences between the mean values of the dose distributions from each scanner were statistically significant (p < 0.05) for all examinations. The variation was greatest for low-dose chest CT, with a greater than fivefold difference in the mean dose values noted between scanners. A very weak correlation was found between dose and scanner age or the number of detector rows. Analysis of image quality revealed no statistically significant differences in any scoring categories, with the exception of the noise category in abdominal imaging. Implementation of the DRLs allowed a reduction in patient dose of up to 41% as a result of a protocol change. Establishing provincial DRLs allows an effective reduction in patient dose without resulting in degradation of image quality.

Effects of vaginal cylinder position on dose distribution in patients with endometrial carcinoma in treatment of vaginal cuff brachytherapy.

PurposeTo investigate the impact of different cylinder positions on dosimetry of critical structures in patients with endometrial carcinoma undergoing three-dimensional image-based vaginal cuff brachytherapy (VCB).Material and methodsWe delivered VCB at a dose of 4 Gy to a depth of 5 mm in the vaginal cuff of 15 patients using three different cylinder positions (neutral [N], parallel [P], and angled [A]) according to the longitudinal axis of the patient. We analyzed the dose-volume distribution and volumetric variability of the rectum and bladder. We converted the total doses to equivalent doses in 2 Gy (EQD2) using a linear-quadratic model (a/b = 3 Gy).ResultsThe mean rectum volume for the N, P, and A positions was 68.2 ± 22.7 cc, 79.3 ± 33.7 cc, and 74.2 ± 29.6 cc, respectively. The mean rectum volume for the P position was significantly larger than that for the N position (p = 0.03). Relative to the N position, the A position resulted in a lower total EQD2 in the highest irradiated 2 cc (D2cc; p = 0.001), 1 cc (D1cc; p = 0.004), and 0.1 cc (D0.1cc; p = 0.047) of the rectum. Similarly, the P position resulted in a lower EQD2 in the D2cc (p = 0.018) and D1cc (p = 0.024) of the rectum relative to the N position. In the bladder, the P position resulted in a higher EQD2 in the D2cc relative to the N position (p = 0.02). There was no dosimetric difference between the P and A positions in either the rectum or the bladder.ConclusionsVaginal cuff brachytherapy in the P and A positions is significantly superior to that in the N position in terms of rectum dosimetry. The bladder dose in the N position is considerably lower than that in the other positions.

A portal dosimetry dose prediction method based on collapsed cone algorithm using the clinical beam model.

Amorphous silicon electronical portal imaging devices (EPIDs) are widely used for dosimetric measurements in Radiation Therapy. The purpose of this work was to determine if a portal dose prediction method can be utilized for dose map calculations based on the linear accelerator model within a commercial treatment planning system (Pinnacle3 v8.0m). The method was developed for a 6MV photon beam on the Varian Clinac 21-EX, at a nominal dose rate of 400MU/min. The Varian aS1000 EPID was unmounted from the linear accelerator and scanned to acquire CT images of the EPID. The CT images were imported into Pinnacle3 and were used as a quality assurance phantom to calculate dose on the EPID setup at a source to detector distance of 105cm. The best match of the dose distributions was obtained considering the image plane located at 106cm from the source to detector plane. The EPID was calibrated according to the manufacturer procedure and corrections were made for output factors. Arm-backscattering effect, based on profile correction curves, has been introduced. Five low-modulated and three high-modulated clinical planned treatments were predicted and measured with the method presented here and with MatriXX (IBA Dosimetry, Schwarzenbruck, Germany). A portal dose prediction method based on Pinnacle3 was developed without modifying the commissioned parameters of the model in use in the clinic. CT images of the EPID were acquired and used as a quality assurance phantom. The CT images indicated a mean density of 1.16g/cm3 for the sensitive area of the EPID. Output factor measured with the EPID were lower for small fields and larger for larger fields (beyond 10×10cm2 ). Arm-backscatter correction showed a better agreement at the target side of the EPID. Analysis of Gamma index comparison (3%, 3mm) indicated a minimum of 97.4% pass rate for low modulated and 98.3% for high modulated treatments. Pass rates were similar for MatriXX measurements. The method developed here can be easily implemented into clinic, as neither additional modeling of the clinical energy nor an independent image prediction algorithm are necessary. The main advantage of this method is that portal dose prediction is calculated with the same algorithm and beam model used for patient dose distribution calculation. This method was independently validated with an ionization chamber matrix.

Evaluation of levels of exposure of adult patients from common radiographic examinations in the Russian Federation in 2009–2014

Diagnostic reference levels are the main and the most effective tools of optimization of the radiation protection of patients from medical exposure. Diagnostic reference levels should be established based on the results of dedicated dose surveys, allowing evaluating typical patient dose distributions in a selected dose quantity for the selected X-ray examinations. The aim of the current study was to assess the distributions of typical effective doses in representative Russian regions. Materials and methods: Typical patient effective doses for the 13 most common radiographic X-ray examinations were collected in 203 X-ray rooms in 101 hospitals in six regions of Russian Federation in 2009–2014. A differentiated approach was used for the estimation of the typical effective doses depending on the image acquisition technology. Effective doses were estimated using «EDEREX» (Russia) computational software. Results and discussion: Results of the dose data analysis indicate the lack of significant differences between the distributions of the typical effective doses between the selected regions, allowing merging the regional samples and further evaluating the pooled (joint) sample. A significant ratio of maximum to minimum (up to two orders of magnitude) due to a presence of X-ray units with abnormally high and low typical effective doses was observed for all 13 selected X-ray examinations. Abnormally high typical effective doses can be explained by performing the examinations using high values of tube current-time product (150–600 mAs) on a maximum field size (up 40×40 cm). Removal of the typical effective doses below 5%-percentile and above 95%-percentile of typical effective dose distributions for all examinations would result in a reduction of a mean effective dose by up to 30% and reduction of a 75%-percentile of the distributions by up to 15%. No significant differences between the distributions of TED for analogue and digital X-ray units were observed for the pooled sample for selected examinations except for the examination of the chest in posterior-anterior projection. Conclusions: These results should be considered in the process of establishing and implementing DRLs as well as in the cost-benefit analysis of the optimization in radiography.